1. Field of the Invention
The present invention relates to a liquid crystal display device using liquid crystal which exhibits a ferroelectric phase when an electric field is applied thereto, and more particularly to a liquid crystal display device having a plurality of pixel capacitors each of which is driven by a plurality of TFTs (Thin Film Transistors).
2. Description of the Related Art
A TN (Twisted Nematic) TFT (Thin Film Transistor) liquid crystal display device is known. The TN TFT liquid crystal display device comprises a substrate on which pixel electrodes and TFTs connected to the pixel electrodes are arranged, a substrate on which a common electrode facing the pixel electrodes is provided, and a chiral nematic liquid crystal which is sealed between these substrates. An alignment of liquid crystal molecules of the liquid crystal is controlled by an interaction between dielectric anisotropy xcex94xcex5 of the liquid crystal and an electric field applied to the liquid crystal.
The TFT liquid crystal display device has been used as a flat-type display device, since its response speed is comparatively fast and a quite high degree of a contrast can be obtained.
The liquid crystal molecules of the TN liquid crystal display device are aligned in a twisted homogenous alignment state in which the liquid crystal molecules are twisted between the substrates and their long axes are aligned almost parallel to the substrates, in a state where no electric field is applied between the substrates. Alternatively, the liquid crystal molecules are aligned in a homeotropic alignment state in which the liquid crystal molecules are aligned almost perpendicular to the substrates, in a state where a sufficiently strong electric field is applied between the substrates.
The TN liquid crystal display device has a problem that the view angle of it is narrow, because such a device displays an image using optical changes corresponding to changes in orientations of the molecules between the homogeneous and homeotropic alignment states in accordance with the intensity of the applied electric field.
For the TN TFT liquid crystal display device, the ratio of the capacitance of a parasitic capacitor between a gate electrode and a source electrode of each TFT to the pixel capacitor which is formed of a pixel electrode, the common electrode and the liquid crystal interposed between these electrodes is large, thus, there is a fluctuation in a voltage of the pixel electrode due to an effect of a gate signal applied to the gate electrode.
To limit the fluctuation, compensating capacitors are connected in parallel to the pixel capacitors.
The formation of the compensating capacitors makes the manufacturing process of the liquid crystal display device complicated. Further, electrodes forming the compensation capacitors cover portions of the pixels so that the aperture ratio of the liquid crystal display device becomes small.
Also well known is a TFT liquid crystal display device, using a chiral smectic liquid crystal having spontaneous polarization, and which exhibits a ferroelectric phase upon the application of an electric field, such as a ferroelectric liquid crystal or an antiferroelectric liquid crystal. This type of liquid crystal display device can operate at a high speed, because the liquid crystal molecules of the liquid crystal are driven by an interaction between the spontaneous polarization and the applied electric field. Because the liquid crystal molecules are driven to vary their directions on an imaginary plane parallel to the substrates, a wide view angle can be obtained.
The ferroelectric liquid crystal or the antiferroelectric liquid crystal has a large spontaneous polarization, and the spontaneous polarization is aligned in one direction in response to the applied electric field . Therefore, the capacitance of the pixel capacitors, which is formed of the pixel electrodes, the common electrode and the chiral smectic liquid crystal having the spontaneous polarization and interposed between these electrodes, is large. In order to charge and discharge the large pixel capacitors, large driving currents and TFTs tolerating such large driving currents are required. To allow the TFTs to tolerate large currents, TFTs need to have large element areas. The effective areas of the pixels are lessened as the element areas of the TFTs expand, and the aperture ratio of the device becomes smaller accordingly.
The present invention has been made in consideration of the above, and an object thereof is to provide a liquid crystal display device with a large aperture ratio and comprising TFTs and a liquid crystal which has a spontaneous polarization and which exhibits a ferroelectric phase at least when an electric field is applied thereto.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a liquid crystal display device comprising:
first and second substrates arranged opposing each other, each of the first and second substrates having the first and second surfaces, and the first surface of the first substrate facing the second surface of the second substrate;
a plurality of pixel electrodes arranged, in a matrix form in both directions of rows and columns, on the first surface of the first substrate;
a plurality of pairs of thin film transistors arranged on the first surface of the first substrate, each pair of the thin film transistors sandwiching a corresponding one of the pixel electrodes in the column direction, and a current path of each thin film transistor having a first end and a second end, and the first end of the current path being connected to a corresponding one of the pixel electrodes;
a plurality of gate lines formed on the first surface of the first substrate, each adjacent pair of gate lines being arranged between two of rows of the pixel electrodes, and parts of the plurality of gate lines serving as gate electrodes of the thin film transistors;
a plurality of data lines formed on the first surface of the first substrate, each data line being arranged aside a corresponding column of the thin film transistors and between two of columns of the pixel electrodes, and being connected to the second ends of the current paths of the thin film transistors of the corresponding column;
a common electrode facing the pixel electrodes and arranged on the second surface of the second substrate; and
liquid crystal, having spontaneous polarization and sealed between the first substrate and the second substrate.
According to the structure above, each pair of TFTs are connected to the corresponding single pixel electrode so that a large current can be supplied to the pixel electrode via these two TFTs. The charging of the pixel capacitors is almost finished during a period of time for selecting each of the pixels. The liquid crystal display device comprising the liquid crystal which has the spontaneous polarization does not require compensating capacitors, because of the large pixel capacitors of the device, so that a satisfactory large aperture ratio can be obtained.
In each columns of the thin film transistors, the second ends of the current paths of two thin film transistors proximate to each other between rows of the pixel electrodes may be connected to the corresponding one of said the lines via a common lead electrode. According to this structure, the area shielded by the current path can be reduced, and the aperture ratio is larger than in that case where the current paths are separately connected to the data lines.
In this case, preferably, each of the thin film transistors comprises a gate electrode, a gate insulating film, and a semiconductor film,
the gate electrode of each of the thin film transistors is formed of a part of the corresponding one of the gate lines,
a gate insulating film is formed on the gate electrode,
a semiconductor film is formed on the gate insulating film so as to face the gate electrode,
the semiconductor film has a first end connected to a corresponding one of the pixel electrodes, and a second end connected to the corresponding one of the data lines via a common lead electrode.
In other words, parts of the gate lines serve as the gate electrodes, and the thin film transistors are arranged on the gate lines. According to this structure, an area covered by the current paths of the transistors can be reduced, and the aperture ratio can be increased.
Preferably, the liquid crystal is chiral smectic liquid crystal exhibiting a ferroelectric phase when an electric field is applied thereto.
The pixel electrodes are rectangular, and each of the thin film transistors is preferably formed along one of two shorter sides of the corresponding one of the pixel electrodes.
The liquid crystal display device may further comprise
a gate driver, connected to the gate lines, for simultaneously applying a gate pulse to each adjacent pair of gate lines which are arranged between two of the rows of the pixel electrodes;
a data driver, connected to the data lines, for applying data signals representing display gradations of a selected row of pixel electrodes to the data lines in synchronized with the gate pulse.
According to the second aspect of the present invention, there is provided a liquid crystal display device comprising:
first and second substrates arranged opposing each other, each of the first and second substrates having the first and second surfaces, and the first surface of the first substrate facing the second surface of the second substrate;
a plurality of pixel electrodes, each having a rectangular shape, and arranged in a matrix form in both directions of rows and columns on the first surface of the first substrate;
a plurality of pairs of thin film transistors arranged on the first surface of the first substrate, each pair of thin film transistors sandwiching corresponding one of the pixel electrodes in the column direction, a current path of each thin film transistor having a first and a second end, and the first end of the current path being connected to corresponding one of pixel electrodes;
a plurality of gate lines for supplying a gate signal to gate electrodes of the thin film transistors, the gate lines being formed on the first surface of the first substrate, each of the gate lines corresponding to one of rows of thin film transistors and being arranged between two of rows of the pixel electrodes;
a plurality of data lines formed on the first surface of the first substrate, each data line being arranged aside a corresponding column of the thin film transistors and between two of columns of the pixel electrodes, and being connected to the second ends of the current paths of the thin film transistors of the corresponding column;
a common electrode opposing the pixel electrodes and arranged on the second surface of the second substrate;
liquid crystal sealed between the first substrate and the second substrate, for forming pixel capacitors by the pixel electrodes and the common electrode, the pixel capacitors having a large capacitance than that of a capacitor arranged between the one end of the current path of each thin film transistor, and the electrodes.
The liquid crystal is preferably smectic liquid crystal having at least smectic layers in which liquid crystal molecules comprise dipole moments in a direction perpendicular to a long axis thereof, and liquid crystal molecules are obliquely aligned toward a normal line of the smectic layer.
Preferably, the liquid crystal has a helical structure of molecules and is made of ferroelectric liquid crystal which has a chiral smectic C phase exhibiting a ferroelectric phase or antiferroelectric liquid crystal which has a chiral smectic CA phase exhibiting an antiferroelectric phase in a state where the liquid crystal is provided between the first and second substrates.
Preferably, the pixel electrodes are rectangular, and each of the thin film transistors is formed along one of two shorter sides of the corresponding one of the pixel electrodes.